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A proposal to turn back climate change by planting vast swathes of land with fast-growing trees and plants that can be burned for electricity, with the carbon they release captured and stored, is not "realistic and feasible," scientists said Thursday.

The idea, included in many economic and science models on how the world might try to reverse runaway climate change, cannot serve as an emergency back-up if the world fails to rapidly switch from fossil fuels to renewable energy and then regrets the consequences, said scientists at the Potsdam Institute for Climate Impact Research in Germany.

"The danger is it's being sold as a realistic and feasible option. But in reality one should think about the decarbonization problem without thinking of this as a real option," said Wolfgang Lucht, one of the authors of a report published Thursday in the journal Earth's Future.

Planting wasteland, "but also large tracts of farmland or natural forests with plants to produce electricity has been regarded as a comparatively safe, affordable and effective way to suck excess carbon out of the atmosphere and reduce runaway climate impacts," researchers said.

Trees and other plants remove carbon dioxide from the atmosphere as they grow. If they are cut and burned, that carbon is then re-released into the atmosphere. But if those emissions are instead captured and stored, the process can reduce the overall levels of carbon dioxide in the atmosphere.

Efforts to remove emissions from the atmosphere look increasingly likely to be needed as the world moves too slowly to end the use of fossil fuels and ramp up renewable energy, something that many scientists now say must happen within the next 20 years to stop global warming from spiralling out of control.

The plantation technology is expected to play a key role in an upcoming Intergovernmental Panel on Climate Change report, due out in 2018, on how the world might hold temperature increases to 1.5 C above pre-industrial levels, an ambition laid out in the Paris Agreement on climate change.

Not ecologically sound

"Climate physicists talk about (the technology), and macroeconomists. Both seem to think that because it's about green stuff it's somehow ecologically sound. But it's not," Lucht said in a telephone interview with the Thomson Reuters Foundation.

"One should not save the climate and ruin (the planet) in the process,"he said.

If carbon emissions are only moderately cut around the world, in line with the pledges countries made as part of the Paris Agreement, using the plantation technology starting in 2050 to try to hold temperature increases to 2 C would require cutting and replanting a third of the world's natural forests or 25 per cent of the world's agricultural land, the report said.

That is likely to put global food security at risk, particularly with the world's population set to grow by 2 billion people by 2050, the report said.

It could also threaten natural forests and biodiversity, increase competition for fertilizers and water "growing plants for fuel would require large-scale irrigation" and create worsening conflict over land, Lucht said.

Some experts also fear that relying too heavily on technological measures to try to pull back emissions if they trigger runaway climate impacts — such as large-scale sea level rise — could both reduce the pressure to end use of fossil fuels now and likely prove ineffective later.

Focus on reduction not removal

"If we continue burning coal and oil the way we do today and regret our inaction later, the amounts of greenhouse gas we would need to take out of the atmosphere in order to stabilize the climate would be too huge to manage,"Lena Boysen, the report's lead author, said in a statement.

The report said much smaller-scale use of carbon-absorbing plantations might make sense in some parts of the world — but only if efforts to slash the use of coal, oil and other fossil fuels are swiftly accelerated now.

"There is no alternative for successful mitigation (of emissions)," the report noted.

Hans Joachim Schellnhuber, director of the Potsdam Institute, said a big range of carbon dioxide removal technologies eventually may become necessary but "rapidly ending fossil fuel use" must be the world's focus.

"In the climate drama that is currently unfolding ... carbon dioxide removal is not the hero who finally saves the day after everything else has failed," he said.